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United States Patent | 5,190,637 |
Guckel | March 2, 1993 |
Inventors: | Guckel; Henry (Madison, WI) |
Assignee: | Wisconsin Alumni Research Foundation (Madison, WI) |
Appl. No.: | 874116 |
Filed: | April 24, 1992 |
Current U.S. Class: | 205/118; 205/125 |
Intern'l Class: | C25D 005/02 |
Field of Search: | 205/118,123 |
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Brochure (in German) "Die LIGA Technik", by MicroParts, Gesellschaft, 1990. E. W. Becker, et al., "Fabrication on Microstructures with High Aspect Ratios and Great Structural Heights by Synchrotron Radiation Lithography Galvanoforming, and Plastic Moulding (LIGA process)", Microelectronic Engineering, vol. 4, No. 1, May 1986, pp. 35-36. W. Ehrfeld, et al., "Fabrication of Microstructures Using the LIGA Process", Proc. IEEE Micro Robots and Teleoperators Workshop, Nov. 9-11, 1987, pp. 1-11. P. Hagmann, et al., "Fabrication of Microstructures of Extreme Structural Heights by Reaction Injection Moulding", International Polymer Processing IV, 1989, pp. 188-195. W. Ehrfeld, et al., "Microfabrication of Sensors and Actuators for Microrobots," Proc. IEEE International Workshop on Intelligent Robots and Systems, Tokyo, Japan, Oct. 31-Nov. 2, 1988, pp. 3-7. W. Ehrfeld, "Three Dimensional Microfabrication Using Synchrotron Radiation," International Symposium on X-Ray Synchrotron Radiation and Advances Science and Technology, Feb. 15-16, 1990, pp. 121-141. W. Ehrfeld, et al., "LIGA Process: Sensor Construction Techniques Via X-Ray Lithography," Technical Digest, IEEE Solid State Sensor and Actuator Workshop, 1988, pp. 1-14. H. Guckel, et al., "Deep X-Ray and UV Lithographies for Micromechanics", Technical Digest, IEEE Solid State Sensor and Actuator Workshop, Hilton Head, S.C., Jun. 4-7, 1990, pp. 118-122. H. Guckel, et al., "Microstructure Sensors," International Election Devices Meeting (IEDM), San Francisco, Calif., Dec., 1990. H. Guckel, et al., "Fabrication of Assembled Micromechanical Components via Deep X-Ray Lithography", Proceedings of IEEE Micro Electro Mechanical Systems (MEMS) 1991, Nara, Japan, 30 Jan.-2 Feb., 1991. W. Meny, et al., "The LIGA Technique--a Novel Concept for Microstructures and the Combination with Si-Technologies by Injection Molding", Proceedings of IEE Micro Electro Mechanical Systems (MEMS) 1991, Nara, Japan, Jan. 30-Feb. 2, 1991, pp. 69-73. PiRL: Polyimide Release Layer, brochure from Brewer Science, Inc. Roger T. Howe, et al., "Silicon Micromechanics: Sensors and Actuators on a Chip", IEEE Spectrum, Jul., 1990, pp. 29-35. J. Mohr, et al., "Fabrication of Microsensor and Microactuator Elements by the LIGA-Process," Proceedings of Transducers '91, San Francisco, Calif., Jun. 24-27, 1991, pp. 607-609. H. Guckel, et al., "Fabrication and Testing of the Planar Magnetic Micromotor," Journal of Micromechanics and Microengineering, IOP Publishing, England, vol. 1, No. 3, Dec. 1991. H. Guckel, "Silicon Microsensors: Construction, Design and Performance," European Solid State Conference, Montreux, Switzerland, Sep. 1991, pp. 387 ` the National Science Foundation (NSF) Grant No. EET-8815285. The United States Government has certain rights in this invention. This invention pertains generally to the field of semiconductor and micromechanical devices and processing techniques therefor, and particularly to the formation of microminiature structures formed of metal. Deep X-ray lithography involves a substrate which is covered by thick photoresist, typically severally hundred microns in thickness, which is exposed through a mask by X-rays. X-ray photons are much more energetic than optical photons, which makes complete exposure of thick photoresist films feasible and practical. Furthermore, since X-ray photons are short wavelength particles, diffraction effects which typically limit device dimensions to two or three wavelengths of the exposing radiation are absent for mask dimensions above 0.1 micron. If one adds to this the fact that X-ray photons are absorbed by atomic processes, standing wave problems, which typically limit exposures of thick photoresist by optical means, become an non-issue for X-ray exposures. The use of a synchrotron for the X-ray source yields high flux densities--several watts per square centimeter--combined with excellent collimation to produce thick photoresist exposures without any horizontal run-out. Locally exposed patterns should therefore produce vertical photoresist walls if a developing system with very high selectivity between exposed and unexposed photoresist is available. This requirement is satisfied for polymethylmethacrylate (PMMA) as the X-ray photoresist and an aqueous developing system. See, H. Guckel, et al., "Deep X-Ray and UV Lithographies For Micromechanics", Technical Digest, Solid State Sensor and Actuator Workshop, Hilton Head, S.C., June 4-7, 1990, pp. 118-122. Deep X-ray lithography may be combined with electroplating to form high aspect ratio structures. This requires that the substrate be furnished with a suitable plating base prior to photoresist application. Typically this involves a sputtered film of adhesive metal such as chromium or titanium which is followed by a thin film of the metal which is to be plated. Exposure through a suitable mask and development are followed by electroplating. This results, after cleanup, in fully attached metal structures with very high aspect ratios. Such structures were reported by W. Ehrfeld and co-workers at the Institute for Nuclear Physics at the University of Karlsruhe in West Germany. Ehrfeld termed the process "LIGA" based on the first letters of the German words for lithography and electro-plating. A general review of the LIGA process is given in the article by W. Ehrfeld, et al., "LIGA Process: Sensor Construction Techniques Via X-Ray Lithography", Technical Digest, IEEE Solid-State Sensor and Actuator Workshop, 1988, pp. 1-4. A difficulty with the original LIGA process is that it can only produce fully attached metal structures. This restricts the possible application areas severely and unnecessarily. The addition of a sacrificial layer to the LIGA process facilitates the fabrication of fully attached, partially attached, or completely free metal structures. Because device thicknesses are typically larger than 10 microns and smaller than 300 microns, freestanding structures will not distort geometrically if reasonable strain control for the plated film is achieved. This fact makes assembly in micromechanics possible and thereby leads to nearly arbitrary three-dimensional structures. See H. Guckel, et al., "Fabrication of Assembled Micromechanical Components via Deep X-Ray Lithography," Proceedings of IEEE Micro Electro Mechanical Systems, January 30-Feb. 2. 1991, pp. 74-79. In principal, it is possible to extend the LIGA process with or without a sacrificial layer by performing several X-ray exposures of photoresist, with electroplating of additional layers of metal after each exposure. However, the extension of the conventional LIGA process encounters topological as well as practical difficulties. The topological problem derives from the fact that electrical contact must be established to the plating base during electroplating. Since the X-ray photoresist is an insulator, electrical contact after X-ray exposure and development of the photoresist can only occur through the previously deposited metal structures which are themselves in contact with the underlying plating base. This implies that an acceptable second X-ray mask topology would have to contain features which are always fully contained within the features of the first mask. Thus, a shaft with steps of ever decreasing diameter could be constructed. However, a shaft having an initial small diameter with a second layer expanding into a larger diameter or into a gear is not possible. The second problem involves the practicality of the process. Exposing and developing the X-ray photoresist and then electroplating produces cracking and crazing of the photoresist. Although it is possible that damage to the photoresist can be partially cured by applying a second photoresist layer, additional layers of photoresist tend to increase photoresist strain and cause geometric deformation of the photoresist. In accordance with the present invention, complex metal structures can be formed in microminiature dimensions utilizing multiple mask exposures which allow substantially arbitrary three dimensional shapes to be formed. These shapes include structures, formed on a substrate, having overhanging portions and tubular structures which can be utilized for hydraulic and pneumatic applications, and parts which are formed on a sacrifical layer to allow complete removal of the parts from the substrate for subsequent assembly. The process is particularly well suited to deep X-ray lithography in which metal structures are formed by X-ray exposure of a relatively thick photoresist and electroplating of metal into the area from which the exposed photoresist is removed, resulting in structures having extremely well defined vertical walls formed to significant height, e.g., 100 to 300 micrometers in thickness. In carrying out the process of the invention, a plating base is initially applied to a substrate, or in appropriate cases, a metal substrate itself may function as a plating base. Where the formed part is later to be partially or wholly removed from the substrate, the plating base is applied over an initial sacrificial layer on the substrate. Photoresist is then cast onto the plating base, and the photoresist is exposed in a pattern, such as from an exposure through an X-ray mask to synchrotron radiation, and the exposed photoresist is removed. A first layer of a primary metal is then electroplated onto the exposed plating base to fill the area defined by the void in the photoresist. The remaining photoresist is then removed and a secondary metal, which can constitute a sacrificial metal, is then electroplated over the previously deposited first layer of primary metal and the plating base. Where the secondary metal is to be utilized as a sacrificial metal, it is selected so that it will be differentially etched by a selected etchent which does not substantially attack the primary metal. The exposed surface of the deposited secondary metal is then machined down to a height which exposes the first metal. The machining, e.g., mechanical grinding or milling, achieves a substantially flat, uniform surface extending across both the primary and secondary metals, and allows the thickness of the first layer of primary metal to be closely controlled, and particularly allows the exposed surface of the primary metal to be formed substantially smooth. In contrast, an electroplated metal by itself before machining shows an uneven surface which is not well controlled in thickness. Moreover, a smooth surface for the first layer of the primary metal is better suited to receive an electroplated second layer thereon than the rough surface of an as-plated metal. The utilization of the secondary or sacrificial metal to completely cover the first layer of the primary metal facilitates machining of the surface of both the primary and secondary metals allowing the surface of the primary metal to be reduced to a desired height, because of the mechanical stability which the secondary metal affords to the primary metal which it surrounds and supports. In contrast, such mechanical machining is difficult or impossible where a polymer photoresist remains adjacent the primary metal; machining generally cannot be done through both the photoresist and the primary metal since the photoresist is relatively weak mechanically and will shred and tear as it is being milled, and the photoresist is not strong enough to provide horizontal support to the relatively small and potentially fragile primary metal structures. Similarly, if the photoresist were removed and machining of the now isolated primary metal structures were attempted, such machining would be extremely difficult and would risk substantial damage such as ripping off portions of the primary metal structure from the substrate. After the first layer of the primary and secondary metals have been machined down to the desired level, a secondary photoresist layer may be applied over both the primary and secondary metal, and then the photoresist may be exposed to X-rays in a desired pattern which may result in photoresist covering at least part of the primary metal and extending out over the secondary metal. The exposed photoresist is then removed and a second layer of primary metal, generally but not necessarily the same metal as the first layer of primary metal, is electroplated in the area of the removed photoresist covering at least in part the first layer of primary metal, but also covering part of the first layer of secondary metal. Because both the primary and secondary metals are conductors of electricity, electroplating can occur over both of these materials. The photoresist is then removed. At this point, if only two layers are desired, the secondary metal can be removed with an etchant which differentially etches the secondary metal and not the primary metal to leave the isolated two layer primary metal structure on the substrate. If this structure is to be freed from the substrate, the plating base is then patterned and removed around the structure and the sacrificial layer under the plating base is then dissolved to free the structure. Alternatively, the first layer of secondary metal may be left in place and a second layer of secondary metal deposited over both the second layer of the primary metal and the first layer of the secondary metal. After electroplating, the exposed surface may then be machined down to reduce the overall height of the multilayer structure to a desired level and to expose the second layer of the primary metal. At this point, the process described above can be repeated, i.e., casting of the photoresist, exposure of the photoresist, removal of the exposed photoresist, and deposit of the third layer of the primary metal. The process can be repeated for as many layers as appropriate. Complex structures, such as hollow tubes and bridge structures, can be readily formed by having the second layer of the primary metal bridge a secondary metal between two or more structures formed of the first layer of the primary metal. Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings. |